During the preparation of Mohr’s salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of Fe²⁺ ion?

The correct answer is dilute sulphuric acid.

Mohr's salt is a double salt with the chemical formula ${\mathrm{(NH}}_4{)}_2{\mathrm{Fe(SO}}_4{)}_2·{\mathrm{6H}}_{2}O$ (Ferrous ammonium sulphate).

During the preparation of its aqueous solution, ferrous ions (${\mathrm{Fe}}^{2+}$) undergo hydrolysis to form ferrous hydroxide (${\mathrm{Fe(OH)}}_2$), which is a turbid colloidal precipitate. The hydrolysis reaction can be represented as:
${\mathrm{Fe}}^{2+}+{\mathrm{2H}}_{2}O\rightleftharpoons {\mathrm{Fe(OH)}}_2+{\mathrm{2H}}^{+}$
To prevent this hydrolysis and keep the solution clear, dilute sulphuric acid ($H_2{\mathrm{SO}}_4$) is added. Dilute sulphuric acid increases the concentration of $H^{+}$ ions in the solution. According to Le Chatelier's principle, the addition of excess $H^{+}$ ions shifts the equilibrium position to the left (the backward direction), thereby preventing the hydrolysis of ${\mathrm{Fe}}^{2+}$ ions.

Dilute sulphuric acid is specifically chosen because it shares a common anion (${\mathrm{SO}}_4^{2-}$) with Mohr's salt, thereby avoiding the introduction of foreign acid radicals (such as chloride from hydrochloric acid or nitrate from nitric acid) which could interfere with subsequent analytical reactions.